The present invention relates generally to control systems, and more particularly to process control systems in a two degree of freedom system.
A process control system implements a controller to shape the response of a process to an input signal. The control system can add gain, time varying properties, frequency components, or a combination of these characteristics to the process signal. By properly choosing these characteristics, the control system can stabilize the response of the process, determine overshoot, on set acceptable error bounds and satisfy other performance criteria.
A two degree of freedom controller is generally implemented in a two degree of freedom system. Such a two degree of freedom system could consist of a setpoint and a disturbance. Within this system, the controller should track the setpoint and reject any disturbances. Controllers of this type, for example, include the precompensator 10 of FIG. 1.
The precompensator 10 of FIG. 1 includes a prefilter 12 and a load controller 14. These two control elements 12 and 14 shape a process input 16 for a process 18. The prefilter 12 shapes a prefilter response 20 to an input variable 22. The load controller 14 shapes the input 16 to the process 18 based on the prefilter response 20 and a process state 30 that is feedback for the system. The process state 30 is altered by a second variable 32 and the transfer function 36 of the second variable 32.
In the configuration of the precompensator 10, the load controller 14 must shape the process input 16 based in part on the prefilter response 20. Any inaccuracies from error in the prefilter 12 are propagated through the load controller 14.
In accordance with a principal feature of the invention, a control structure comprises a first controller, a second controller, and a process model. The first controller and the process model are configured in a partitioned feedback loop to receive input from a first variable. The second controller is configured in parallel with the first controller to receive input from a partitioned feedback loop and feedback from a process. The process receives input from a second variable.
In accordance with another principal feature of the invention, the control structure comprises the first controller, the second controller, and the process model. The first controller shapes a response to the first variable. The process model predicts the output of the process. The second controller is isolated from the first input and receives its input from the predicted output and feedback from a process output. The second controller shapes a response to an error between the predicted output and the measured output.